Chemical weapons are not banned because they are indiscriminate. All indiscriminate use of weapons is technically banned. So while indiscriminate use is illegal, it was not the crux of the reason for their being banned.

Even using conventional artillery rounds on a village indiscriminately is a war crime. While I agree it is easier to hit a military target with artillery, you will face war crimes charges if you don't do it properly. I think the indiscriminate effects of chemical weapons is a factor, but they were banned quite a bit before the rest of our modern war crimes laws; the logic used today did not necessarily apply to when they were first banned.

Chemical weapons are specifically banned because of the suffering of the people on the receiving end. It maims people, leaving them to suffer horrible deaths, or worse, lives dealing with the aftermath. Countries saw first-hand the non-lethal effects on soldiers and they decided it was not worth using chemical weapons that will blind/cripple tens of thousands of people for life. It goes against the concept of waging a just war and was deemed to just cause unacceptable levels of suffering.